As a follow-up, I tried using Happich Simichrome Polish…
http://www.simichrome-polish.com/
… on the headlamp lens of my ’01 ML430. The results are stunning. It did take 3 or 4 applications to achieve a clear lens again, but because the lenses are plastic, I preferred multiple apps so as not to scratch the lens. My lenses were marginally dull with only a hint of yellow. They are now clear. I suppose heavily yellowed or translucent lens may be too far gone. However, as a pro-active and preventive measure, I plan to polish the plastic headlamp lenses every 6 months or so. Thanks
TheDon for the suggestion!
Cheers,
Jeff